This invention relates to an air supply system for a supercharged internal combustion engine, and more particularly to an arrangement for increasing the boost power at low and mid ranges without having excess boost at high speed conditions.
It has been proposed to increase the power of an internal combustion engine by employing some form of supercharging for compressing the air charge delivered to the combustion chambers of the engine. Most superchargers used for this purpose provide a boost that increases as the engine speed increases. Many of the superchargers used for this purpose also have a non-linear increase in boost pressure with relation to engine speed such as centrifugal type of superchargers which may be either engine driven or driven by an exhaust turbine (turbochargers). As a result, supercharged engines tend to have a high degree of power at maximum engine speed but their mid range and low speed performance may not be significantly greater than normally aspirated engines. Although these problems can be offset by designing the supercharger to provide higher boost pressures at low and mid range, then excessive boost is generated at high speed and performance can be deteriorated and/or the engine life significantly reduced.
It has been proposed to limit the maximum boost pressure of a supercharger by somehow reducing the speed at which the supercharger is driven as the engine speed increases. With a turbocharger this is done by using a relief valve in the exhaust turbine which opens when the exhaust pressure reaches a certain limit so as to reduce the speed at which the turbocharger is driven. Similar devices may also be employed with positively driven superchargers. Another alternative is to provide a fixed throttle in the discharge from the supercharger so as to reduce the maximum pressure which can be generated in the induction system.
Both of the expedients mentioned have a number of disadvantages. For example, reducing the speed at which the supercharger is driven, for example by using a pressure relief valve, provides a number of disadvantages. First, these relief valves are subjected to very high temperatures and failure rates can be quite high. In addition, the devices can add to the noise in the exhaust system and also add significantly to the cost of the over all engine.
Fixed throttles, on the other hand, tend to reduce the performance not only at high speed but also at low speed and thus are not totally satisfactory.
It is, therefore, a principal object of this invention to provide an improved and simplified arrangement for permitting a supercharger to generate high boost pressures at low and mid range speeds without creating excess boost at high speeds.
It is a further object of this invention to provide an improved and simplified arrangement for reducing the boost pressure generated by a supercharger at high speeds without adversely affecting the boost pressure at low speeds.
It is yet a further object of this invention to provide an improved arrangement for controlling the boost pressure of a turbocharged engine.